Transparent conductive films (transparent electrodes) produced by coating a glass substrate and a PET (polyethylene terephthalate) resin film with ITO (indium tin oxide) by sputtering, vacuum deposition, or the like have been used so far. However, indium contained in ITO is a rare earth, and there is uneasiness at procurement thereof and there is a fear that dusts of ITO generated at a production site may cause injuries to health. Therefore, alternatives thereof are actively under research and development. Graphenes are expected as a promising candidate, but have not yet put into practical use because they could not be produced stably in an industrial scale.
In lithium ion batteries, capacitors, lithium ion capacitors and fuel cells, film-like electrodes, which are produced by mixing fine powders of graphite, carbon black or the like with polyvinylidene fluoride and a solvent to form a slurry and coating and drying the slurry on a current collector such as a copper foil, have been put into practical use. For example, film-like electrodes, which are produced by using a copper foil and artificial graphite and natural graphite as main materials have been put into practical use on negative electrodes for lithium ion batteries. Such lithium ion batteries are put into practical use as batteries for driving of electric vehicles and are under active research and development since improvement of energy efficiency and increase in a cruising distance per one charge are required.
Graphite is an active material for producing charging and discharging by an intercalation reaction of a lithium ion, but it is difficult to form graphite alone into a sheet since graphite is a powder. For that reason, graphite is adhered together with a binder such as polyvinylidene fluoride to a copper foil or the like to be formed into a sheet. Though there is a graphite sheet material formed into a sheet using graphite alone, mechanical strength thereof is very low in its bending direction and pulling direction, and therefore, use of such a material is limited to a heat resistant packing, etc., in which a compressive strength is important.
Multi-layer graphenes are promising as negative electrodes of lithium ion batteries and electrodes of lithium ion capacitors because they produce intercalation of lithium ion like graphite. For example, if graphene film can be formed directly on a copper foil, electric resistance which is a problem in use of a binder component can be decreased, and in addition, an electrode layer can be made thinner and a charge and discharge capacity is increased. Further, if graphene film can be formed directly on a resin film or graphene can be formed into a complex with a resin film, an electrode can be formed without using a copper foil. In lithium ion batteries for electric vehicles, making a weight thereof light is required, but a specific gravity of copper is as heavy as approximately 9 g/cm3. Meanwhile, if a graphene film can be formed on a PET resin film or a composite of graphene with a PET resin can be produced, a specific gravity thereof is decreased to less than 2 g/cm3, which can lead to decrease in weight to a large extent.
There are a CVD method, an SiC thermal decomposition method, a graphene oxide reducing method, etc. as a technology for producing graphenc on a substrate. However, each of them has respective problems. Therefore, a method for producing more easily and stably is demanded.
For example, a CVD method is such that a film comprising graphene is formed on one surface of a metal foil such as copper or nickel by chemical vapor deposition (CVD), then after laminating a resin film on the graphene film, the metal foil is removed by etching and the graphene film is transferred from the etched surface onto an end substrate of PET resin or the like (Non-patent Documents 1 to 4). However, in such a method, since a composite material, in which a graphene film is strongly bonded to a metal foil due to a catalytic action of the metal foil, is produced, in order to transfer the film once formed on the metal foil to a substrate, all metal foil need to be removed using an acid, which makes a production process complicated and results in a problem such as a fear of causing a defect on the film in a transferring process.
Further, the SiC thermal decomposition method is a method for heating an SiC substrate to around 1300° C. to remove Si from its surface and recombine the remaining C into graphene autonomously (Non-patent Document 5). However, there is a problem such that the SiC substrate to be used is expensive and transferring from the SiC substrate is difficult.
Furthermore, the graphene oxide reducing method is a method for oxidizing a graphite powder, dissolving the oxidized graphene powder in a solution, applying the solution to a substrate and then reducing the oxidized graphene (Non-patent Documents 6 and 7). However, since a reducing process is necessary and it is difficult to complete the reduction, there is a problem such that it is difficult to secure enough electric conductivity and transparency.